Wireless power (WP) transfer systems use the mutual inductance between two magnetic coils to transfer power through magnetic induction. At the receiver side, usually a receiver coil is connected to a bridge rectifier following by a regulator. The bridge rectifier converts the AC power signal to DC power supply and the regulator regulates the DC power supply to a suitable voltage level for the following circuit such as a battery charger. Wireless power systems are commonly classified as either “inductive” or “resonant” type. In inductive type wireless powering system, a wireless transmitter and receiver operate like a tightly coupling transformer to deliver energy. The restriction in inductive type makes it only suitable for single receiver charging at the same time. On the other hand, in resonant type wireless powering system, power delivery is through a loosely coupled coil pairs and by utilizing electrical resonance to enhance the system efficiency. Receiver numbers can be increased and charged in the same field.
At the wireless power receiver side, voltage regulation is applied to step-down the rectifier voltage to a suitable voltage for the following charger circuit. In inductive single receiver wireless power system, this regulation can be a linear Low dropout regulator (LDO). The efficiency of a LDO is defined by its output-to-input ratio. In a single receiver wireless system, the LDO input voltage (the rectifier voltage) can be controlled to very close to its output voltage and get higher power efficiency. Power control is by sending power control message from the receiver to the transmitter through in band or out-of-band communication.
In resonant mode wireless power system, multiple receivers make it impossible to control all the rectifier voltages close to the target charging voltage because each receiver have different coil coupling factors. The rectifier voltage thus can be much higher than the regulator output that make the power transfer very inefficient through an LDO. Therefore, a switching mode regulator (SMPS) is applied for better efficiency when the voltage step down ratio is large.
Recently, fast charging is more and more important for smart phone and tablet applications. Reducing the charging time with larger charging current (e.g., >1 A) is adopted by more and more products that already launched in the consumer market. In fast charging, the charger circuit can charge at a higher input voltage (e.g., ˜20V) rather than a regulated voltage (e.g., ˜5V). As a result, the wireless power receiver can directly connect the rectifier output to the fast charging charger through a power switch (PSW). The power switch is used to control the start/stop of wireless charging that is required by some wireless power standard.
In a multi-mode wireless receiver IC that aims to support both inductive and resonant type with the fast charging function, it requires large die area to implement the pass device of LDO, SMPS and PSW separately and make the IC implementation costly. A more cost effective method is to implement the LDO, SMPS and PSW by sharing the same pass device. Furthermore, to achieve high power transfer efficiency performance, using NMOS type FET as its pass device has better efficiency and smaller die area than PMOS type FET pass device.
Implementing the control circuit for sharing the NMOS pass device of LDO, SMPS and PSW requires non-trivial biasing configuration. Bootstrapping technique is well known for the implement of high-side driver of SMPS with NMOS pass device. In PSW mode or a near dropout operating LDO mode, it requires a step-up voltage for powering the LDO and PSW controller. This step-up voltage can be implemented by an on-chip charge pump circuit.
A solution for providing a multi-mode wireless receiver IC that supports both inductive and resonant type with the fast charging function, reduced cost, and improved efficiency is sought.